Method and apparatus for sealing wells in co2 sequestration projects

ABSTRACT

Method for forming a downhole plug within an abandoned well casing for preventing the flow of gas through the plug and reaching the surface, particularly for CO2 sequestration projects. A milling tool mills out a longitudinal section of the perimeter of the well casing and surrounding cement and detritus to expose the well bore. A heater containing solid metallic alloy is lowered into the casing and the alloy is melted so that it fills the milled out area and flows into the face of the well bore. The heater is removed to allow the alloy to solidify. The upper casing acts to restrain any movement of the plug following solidification of the alloy.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application Ser. No. 61/106,936 filed Oct. 20, 2008 and entitled METHOD AND APPARATUS FOR SEALING WELLS IN CO2 SEQUESTRATION PROJECTS.

INTRODUCTION

This invention relates to a method and apparatus for the sequestration of carbon dioxide and, more particularly, to the sequestration of carbon dioxide from large fossil fuel burning facilities.

BACKGROUND OF THE INVENTION

To prevent carbon dioxide (CO2) from entering the atmosphere when large fossil fuel burning facilities are used, a proposed solution is to capture the CO2, concentrate and compress it and transport it to a depleted oil or gas field which is then used as a storage vessel facility. The compressed CO2 is injected into the depleted and depressurized geologic formation previously holding oil or gas. The process is capital intensive and time consuming and it clearly is only of value if the storage is permanent and there is no leakage from the reservoir where the CO2 is injected. Fields that may be satisfactory to hold the CO2 have defining characteristics which include being closed and with no natural fractures or permeable cap rock. These depleted fields also contain numerous wells that have been abandoned following depletion. The usual procedure of dealing with such abandoned wells includes plugging the wells with cement and capping the well casings near the surface with a sealed steel plate. The wells will have been drilled through the cap rock of the storage reservoir so they too must be permanently sealed or they will become conduits for the injected CO2 to lead to the atmosphere which is undesirable.

The use of cement which is commonly used in the completion of wells and sealing them when they are subsequently abandoned is not a reliable procedure to prevent leakage of gas under pressure. This is so particularly with older wells which have used cement with less sophisticated cement formulations than those formulations presently available. The unreliability of such abandonment procedures has let to a proposed change in well abandonment procedure being mandated by the Alberta Energy Resources Conservation Board. This proposed change now requires that abandoned well caps not be sealed so as to avoid the safety problems resulting from accidental damage of a abandoned well casings and the inadvertent release of flammable natural gas which has accumulated below the well cap as a result of gas diffusion through sealing cement which has become permeable over time. The cement further does not form a particularly tight seal against the steel wall of the well casing and cracks can develop from a variety of causes that then act as conduits for gas flow through towards the surface. Well cement in contact with CO2 and water or brine is also vulnerable to damage and deterioration through chemical reaction.

Yet a further disadvantage with the use of cement plugs in well abandonments relates to long term degradation of the steel well casing. Over time, corrosion of the steel casing is likely occur within a certain period. The corrosion can again allow undesirable gas defusion to the surface. To prevent this possibility, it has been suggested that the steel casing be removed following well abandonment and that a reliable gas seal be deployed within the open well bore.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a method of forming a metal alloy plug within the well casing of a well bore of an abandoned oil or gas well to prevent the flow of gas through said metal alloy plug comprising the steps of dropping a casing milling tool down said well casing to a predetermined depth within said well casing, said well casing having a plug below said predetermined depth, milling out a longitudinal section of said well casing and the well cement and detritus surrounding said well casing and above said plug so as to expose the wall of said well bore while leaving the upper portion of said well casing above said milled out area and while leaving the bottom portion of said well casing below said milled out area, removing said casing milling tool and dropping a heater into said well casing to said predetermined depth, said heater carrying a solid metallic alloy plugging material, increasing the temperature of said heater to melt said metallic alloy and maintaining said increased temperature until said melted metallic alloy has flowed from said heater and permeated said well bore to a desired depth, removing said heater from said well casing and allowing said melted metallic alloy to solidify within said well bore, said casing and said milled out area.

According to a further aspect of the invention, there is provided apparatus used to form a metallic alloy plug within a well casing, said apparatus comprising a milling tool used to mill out a longitudinal section of a well casing at a predetermined depth within said well casing and leaving the upper and lower portions of said well casing above and below said milled out area and a heating tool operable to carry solid metallic alloy billets, to melt said billets at an elevated temperature of said heating tool to form liquid alloy and to maintain said elevated temperature of said heating tool until said liquid alloy flows into said milled out area for a desired time period, said heating tool having a connected power cable to supply power to said heating tool and to lower and raise said heating tool within said well casing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Specific embodiments of the invention will now be described, by way of example only, with the use of drawings in which:

FIG. 1 is a diagrammatic side cross-sectional view of a cement plugged well with a window milled through the casing and the cement sheath surrounding the casing;

FIG. 2 is a diagrammatic side cross-sectional view of a fusible alloy heater lowered within the well to the milled window;

FIG. 3 is a diagrammatic side cross-sectional view of the molten alloy following heating which extends through the milled window and into the formation; and

FIG. 4 is a diagrammatic side-cross sectional view of the solidified alloy plug extending through the window and into the formation with the alloy heating tool withdrawn from the well.

DESCRIPTION OF SPECIFIC EMBODIMENT

It is useful to select an interval for forming the gas sealing plugs in the abandoned wells. Several sites for forming the gas sealing plugs may be selected with several plugs set. The formation of only one such plug will be described herein, it being understood that such a process can apply to several other sites that may be considered favorable for the sealing process.

Conveniently, a favorable site for a sealing plug within a well 105 will be in an area adjacent a permeable stratum atop an impermeable stratum such as cap rock such as that site generally illustrated at 100 in FIG. 1. The cap rock 101 will seal the top of the reservoir into which the CO2 is injected.

A section of the casing 102, conveniently less than one (1) meter in length, is milled from the casing string 103. The annulus surrounding the casing is cleaned of cement 104 and other foreign material such that the face 110 of the well bore is exposed. FIG. 1 illustrates the condition of the target well at this stage of the procedure. Generally, such a well 105 will have cement plugging material 111 within the well casing 102 at the bottom of the milled section 102. In the event, however, that there is no such cement below the milled section, a bridge plug (not illustrated) is set just below the bottom of the milled section 102 as is known.

A heating tool 112 (FIG. 2) is attached to the end of an electrically conducting cable 113 and is lowered from the surface down the well casing 103 to the top of the plugged casing. The heating tool 112 is cylindrical and hollow with a retaining flange 114 near its bottom. Electrical heating elements (not shown) are embedded within the wall of the tool 112 and are connected to an electrical power source 120 on the surface by the electrical conducting cable 113. Billets 121 of solid fusible alloy are inserted within the hollow heating tool 112 before it is lowered down the well 105 which will be normally filled with water, brine or drilling mud to the ground surface. Such a heating tool is disclosed in our U.S. Pat. No. 7,065,607, the contents of which are herein incorporated by reference.

When the heating tool 112 reaches the top of the plugged casing adjacent the milled window 102, electrical power is applied through the power cable 113 to the heating elements within the tool 112. The temperature of the elements rises above the melting point of the contained fusible alloy and the molten metal flows out of the bottom of the heating tool 112 and into the well bore 110 and casing 103 as seen in FIG. 3. For a predetermined period, power continues to be applied to the heating elements to raise the temperature of the well bore 110 and the surrounding formation material above the melting point of the metal alloy. Pressure is then applied to the fluid column within the well bore 110 above the level of the molten alloy. The pressure assists in forcing the molten alloy into the porosity of the geologic stratum.

The power is then shut off to the heating tool 112 and the tool 112 is lifted from the well bore 105 and removed as seen in FIG. 4. The molten alloy cools by conduction of heat to the surrounding earth and the alloy solidifies. The alloy plug 122 that remains fills the milled section 102 of casing 103, covers the end of the production casing 103 and its cement sheath 104 and renders the surrounding geologic formation impermeable to gas flow by virtue of the solid alloy that fills its porosity to a depth to effect a seal against gas flowing upward from below. By filling the entire milled section 102 of the casing 103 with solidified fusible alloy 122, the alloy plug 122 butts against the edge of the well casing 123 above and cannot move as long as the casing 123 remains competent. The well above the alloy plug 122 may be filled with cement or some other suitable material (not shown) as ballast to over balance against any subsequent pressure from below that otherwise might cause a creeping movement of the alloy plug 122. The above described alloy plugging procedure may be repeated as many times as desired in different permeable geologic strata in the open well bore in order to increase the assurance that a permanent gas seal has been assured.

In respect of the alloy intended to be used as the alloy plug 122 for the plugging procedure, certain properties are conveniently exploited to make the procedure more efficacious. First, the alloy must conveniently expand volumetrically upon solidification from the liquid phase sufficiently to form an effective gas seal against corroded, oily, and dirty steel casing 103 as well as within the confined space of the well annulus exposed by the milling of the casing section; secondly, in the liquid state, the alloy must conveniently have a sufficiently low viscosity and surface tension in order to be successfully pressured into the permeable geologic stratum; third, the alloy must conveniently have a specific gravity high enough to displace efficiently any fluid material present such as water, brine, or drilling mud; fourth, the alloy must not chemically corrode significantly in contact with CO2 or other acid gas and saltwater; fifth, the alloy must not itself corrode significantly, nor can it cause significant corrosion of the steel well casing due to galvanic action; and, sixth, the solidified alloy should be impermeable to gas flow under pressures common to gas sequestration operations.

There are further desirable properties associated with the alloy. First, it is preferred if the alloy is comprised of component metallic elements that are not toxic if the sealing plug 122 may have the possibility of contact with fresh ground water; second, the alloy is conveniently a eutectic mixture such that the alloy melts and solidifies at a single characteristic temperature; and, third, the melting point temperature of the alloy should be low enough to allow efficient melting with a heater 112 deployed within the well without causing damage to the well bore face 110 within which the molten alloy flows at the pressure applied.

There are four known substances that expand volumetrically upon solidification from the liquid to the solid phase. The substances are water, and the metallic elements bismuth, antimony, and gallium. To be useful as a component in a well sealing plug, a substance must be solid at temperatures existing in wells. This eliminates water and gallium. Gallium melts at 29.6° C. and is also corrosively attacked by inorganic acids. Antimony is possibly useful as an alloy component but it is disadvantageous in that it is toxic.

Bismuth is not toxic and a number of its alloys expand volumetrically upon solidification from the liquid phase. Some bismuth alloys are eutectic mixtures that melt at temperatures convenient for in situ formation of plugs in wells. Bismuth and its alloys generally have low liquid phase viscosities and surface tensions and specific gravities high enough to efficiently displace fluids likely to be present in wells. Bismuth alloys are furthermore impermeable to gas flow under pressure conditions of CO₂ injection wells.

While there are several bismuth alloys which may be useful to form well plugs, an alloy known as CERROTRU (Trademark) has been shown to be most desirable under many well conditions of pressure and temperature. It is comprised of bismuth and tin, neither of which are toxic. It is also a eutectic mixture which melts and solidifies at a temperature of 137 deg. C. Tests have shown that the CERROTRU alloy of bismuth and tin does not itself corrode significantly under the physical and chemical conditions of oil, natural gas, and CO₂ injection wells. In addition, the presence of this alloy in contact with well fluids and steel well casing does not cause corrosion of the casing steel. Bismuth is essentially inert under well conditions, and the behavior of CERROTRU alloy approximates that of elemental tin. Under acidic conditions, tin passivates and does not corrode. Under the anaerobic well environment, the passivated tin galvanic couple with steel causes the steel to passivate also and no increased corrosion of the steel results.

Many modifications will readily occur to those skilled in the art to which the invention relates and the particular embodiments described herein should be taken as illustrative of the invention only and not as limiting its scope as defined in accordance with the accompanying claims. 

1. A method of forming a metal alloy plug within the well casing of a well bore of an abandoned oil or gas well to prevent the flow of gas through said metal alloy plug comprising the steps of dropping a casing milling tool down said well casing to a predetermined depth within said well casing, said well casing having a plug below said predetermined depth, milling out a longitudinal section of said well casing and the well cement and detritus surrounding said well casing and above said plug so as to expose the wall of said well bore while leaving the upper portion of said well casing above said milled out area and while leaving the bottom portion of said well casing below said milled out area, removing said casing milling tool and dropping a heater into said well casing to said predetermined depth, said heater carrying a solid metallic alloy plugging material, increasing the temperature of said heater to melt said metallic alloy and maintaining said increased temperature until said melted metallic alloy has flowed from said heater and permeated said well bore to a desired depth, removing said heater from said well casing and allowing said melted metallic alloy to solidify within said well bore, said casing and said milled out area.
 2. Method as in claim 1 and further comprising applying pressure on said liquid metallic alloy following said melting of said solid metallic alloy and allowing said pressure to assist said melted alloy to flow from said heater.
 3. Method as in claim 2 wherein said metallic alloy comprises bismuth as a principal component.
 4. Method as in claim 1 wherein the quantity of melted metallic alloy is sufficient to entirely fill said milled out area between said upper and lower portions of said well casing, said upper well casing restraining movement of said metal alloy plug following said solidification of said metal alloy within said milled out area.
 5. Apparatus used to form a metallic alloy plug within a well casing, said apparatus comprising a milling tool used to mill out a longitudinal section of a well casing at a predetermined depth within said well casing and leaving the upper and lower portions of said well casing above and below said milled out area and a heating tool operable to carry solid metallic alloy billets, to melt said billets at an elevated temperature of said heating tool to form liquid alloy and to maintain said elevated temperature of said heating tool until said liquid alloy flows into said milled out area for a desired time period, said heating tool having a connected power cable to supply power to said heating tool and to lower and raise said heating tool within said well casing.
 6. Apparatus as in claim 5 and further comprising means to apply pressure on said liquid alloy to assist flow of said liquid alloy from said heating tool and into said milled out area. 